Multimodal imaging is highly desirable for accurate diagnosis because it can provide complementary in-formation of each imaging modality. In this study, multifunctional Fe3O4/TaOx core/shell nanoparticles were synthesized using a sol-gel reaction of tantalum (V) ethoxide in a microemulsion containing Fe3O4 nanoparticles. The resulting nanoparticles were biocompatible and exhibited prolonged circulation time. When the nanoparticles were intravenously injected, the tumor-associated vessel and the microenvironment of the tumor were observed with computed tomography (CT) and magnetic resonance imaging (MRI), respectively.